Exoplanets’ Unstable Orbits Could Be Due to Over Tilting

2 min

For almost a decade, thanks to an increased effort in exoplanet hunting such as NASA’s Kepler mission, astronomers have identified a lot of Earth-like planets outside our solar system. However, they had a problem explaining why a significant number of these exoplanets, usually in pairs, have unstable orbits. There seemed to be an invisible force pushing them apart from each other.

A team of Yale researchers thinks they might have found an answer to that mystery: according to their calculation, the pole of these planets could be over-tilted.

In astronomy obliquity, also known as axial tilt, describes the angle between a body’s rotational axis and its orbital axis. All planets exhibit axial tilt to a certain degree. Currently, Earth has an axial tilt about 23°, while the one of Mars is 25°. An oddball in the solar system, Uranus has an axis tilts of 82°, making it look like a tipped-over barrel that rotates on its side.

Scientists had previously suspected that the tides on these planets, caused by their host star, could nudging them out of the regular orbits by draining their orbital energy. But the problem is that later calculation revealed the tides aren’t strong enough to pull off such a feat.

Sarah Millholland and professor Gregory Laughlin, the Yale astronomers behind the latest study, added their ingenious tweak to the original theory: they proposed that if these exoplanets have a substantial obliquity, similar to the case of Uranus, then the tides would have sufficient kinetic energy to affect their orbit.

In a press release, Millholland explained their idea: “When planets such as these have large axial tilts, as opposed to little or no tilt, their tides are exceedingly more efficient at draining orbital energy into heat in the planets. This vigorous tidal dissipation pries the orbits apart.”

The probable over-tilting feature of these exoplanets will have board implications in the planet’s physical characters, such as their climate and atmosphere.

Earth’s current obliquity allows our planet to have a gradual switch between distinct seasons. Hypothetically, if a planet has a perfect 90° tilt angle, it would always be summer and day time at one pole, and winter and night time at the other.

And also thanks to the Moon, which has a stabilizing effect on Earth’s obliquity, our poles only oscillate in a small range (22°-24°) in the past 5 million years. Otherwise, scientists suspect that Earth’s obliquity might reach near 90° over several billion years.

For their next step, the astronomy duo will be looking into the effect of substantial obliquity on planets’ structures over time.